Process for polymerizing vinyl chloride using a reflux condenser

ABSTRACT

FFIG-01 THIS INVENTION PROVIDE AN IMPROVED PROCESS FOR POLYMERIZING VINYL CHLORIDE IN AN AQUEOUS MEDIUM USING A REFLUX CONDENSER. THE IMPROVEMENT COMPRISES (1) DURING THE CONDENSATION PERIOD SPRAYING A COATING OF WATER ON THE INNER SURFACES OF THE COMDENSER TO INHIBIT THE FORMATION THEREON OF POLYMERIZED VINYL CHLORIDE PARTICLES, AND (2) SPRAYING WATER IN THE PATHOF AND IN A DIRECTION COUNTER-CURRENT TO THE FLOW OF VINYL CHLORIDE VAPORR AS IT PROGRESSES FROM THE REACTOR TO WITHIN THE CONDENSER THEREBY TO ASSIST IN CONDENSING THE VINYL CHLORIDE FOR RETURN TO THE REACTOR. THE SPRAY MAY INCLUDE A DEFOAMER TO REDUCER THE TENDENCY OF CARRYOVER OF POLYMER INTO THE CONDENSER TUBES.

July 23, 1914 w, j -R ETAL 3,825,512

PROCESS FOR POLYMERIZING VINYL CHLORIDE USING A REFLUX CONDINSEZ? FiledSept. 11, 1972 United States Patent Office Patented July 23, 19743,825,512 PROCESS FOR POLYMERIZING VINYL CHLORIDE USING A REFLUXCONDENSER William M. Reiter, Mendham, James E. Cooper, Dover,

and Krishnakant K. Sheth, West Caldwell, N.J., asslgnors to SumitomoChemical Company, Ltd., Osaka, Japan, and Universal PVC Resins, Inc.,Painesville, Ohio, fractional part interest to each Continuation-impartof abandoned application Ser. No. 222,975, Feb. 2, 1972. Thisapplication Sept. 11, 1972, Ser. No. 288,176

Int. Cl. C0813 U1] US. Cl. 260--92.8 W 11 Claims ABSTRACT OF THEDISCLOSURE This invention provides an improved process for polymerizingvinyl chloride in an aqueous medium using a reflux condenser. Theimprovement comprises (1) during the condensation period spraying acoating of water on the inner surfaces of the condenser to inhibit theformation thereon of polymerized vinyl chloride particles, and (2)spraying water in the path of and in a direction counter-current to theflow of vinyl chloride vapor as it progresses from the reactor to withinthe condenser thereby to assist in condensing the vinyl chloride forreturn to the reactor. The spray may include a defoamer to reduce thetendency of carryover of polymer into the condenser tubes.

This application is a continuation-in-part of our previously filedco-pending application U.S. Ser. No. 222,- 975, filed Feb. 2, 1972, nowabandoned.

BACKGROUND OF THE INVENTION This application relates to polymerizingvinyl chloride and more particularly refers to an improved process forpolymerizing vinyl chloride in an aqueous medium using a polymerizationreactor and reflux condenser.

Commercially, polymerization of vinyl chloride monomer is carried out inbatches in large reaction vessels of capacity of the order of2,000l0,000 gallons which vessels are surrounded by a water jacketthrough which either hot or cold water is passed to heat or cool thereaction contents. Polymerization of vinyl chloride is a highlyexothermic reaction and generates a great deal of heat. It is essentialthat the temperature be regulated within narrow limits in order toproduce the desired product. This has been accomplished by passing waterthrough the jacket surrounding the reactor to maintain the temperature.It will be evident that the amount of surface area available for coolingthe reactor is relatively small compared to the contents of the reactor.A vessel having a volumetric capacity of 5,000 gallons has a coolingsurface area of approximately 350 sq. feet. Reaction vessels aregenerally glass lined, and this combined with the thick outer steelshell makes for poor heat transfer. Normally reactor vessels of thistype have a heat transfer rate during polymerization of about 30-50B.t.u. per hour per sq. foot per degree Fahrenheit.

Reflux condensers were suggested for use in conjunction with reactionvessels many years ago. Reflux condensers can be employed having a muchhigher surface area, up to twice or more the surface area of the coolingsurface area of the reactor. Of at least equal importance is the factthat the heat transfer coeflicient of the condenser is appreciablyhigher than that of the reactor surface, i.e., of the order of 80 to 100B.t.u. per hour per sq. foot per degree Fahrenheit. In short, the refluxcondenser provides means for extracting 2-5 times or more depending onsize of condenser as much heat as is feasible from the surface of thereactor. The advantage of extracting a greater amount of heat in ashorter period of time is self-evident in that one can carry out thepolymerization in an appreciably shorter interval of time which meansgreater capacity, lower investment cost for the same capacity, andsmaller operating expense for the same capacity.

Despite proposals made in the past to use a reflux condenser inconnection with the polymerization of vinyl chloride in an aqueousmedium in order to reduce the required polymerization time, most of thecommercial processes to produce vinyl chloride polymer do not employ areflux condenser. In those instances where a reflux condenser is usedspecial provisions are required, such as lowering the batch level in thereactor with resultant re duction in capacity. A primary reason for notusing a reflux condenser in polyvinyl chloride production is fouling dueto deposit of vinyl chloride polymer on the interior surface of thecondenser which requires frequent cleaning and the time, labor, anddifficulties involved in cleaning oflsets the reduction inpolymerization time.

SUMMARY OF THE INVENTION The present invention relates to a process forpolymerizing vinyl chloride in an aqueous medium within a reactorwherein vinyl chloride vapor is released from the reactor, condensed ina condenser, and returned as a liquid to the reactor, and isparticularly directed to the feature which comprises contacting theinterior surface of the condenser with water, whereby deposit of vinylchloride polymer on the interior surface is substantially reduced.Another feature is to contact the vinyl chloride vapor as it iswithdrawn from the reactor and before it enters the condenser tubes,with a spray of liquid containing a defoamer, thereby reducting thetendency of carryover of polymer into the condenser tubes.

In the preferred method of operation a portion, about 20-45 percent, ofthe water of the aqueous medium normally charged to the reactor isemployed as the water introduced into the condenser during vaporizationand condensation of vinyl chloride vapor during the polymerization ofthe monomer in the reactor.

Because the reflux condenser facilitates a much more rapidpolymerization, reactor capacity can in most cases be doubled andeconomics of manufacturing are much more attractive. Furthermore, thereflux condenser process does not adversely aflect the properties of theproduct or polymerization parameters such as conversion and yield. Theneed for condenser cleaning and time required for such cleaning are bothreduced. Hence, on-stream factor for the condenser is increased andmaintenance costs are reduced.

As appreciated by those skilled in the art, when a reflux condenser isemployed, the increase in heat-removal capacity provided by thecondenser permits an increase in the concentration of initiator overthat normally used when a reflux condenser is not employed, the rate ofpolymerization and heat of reaction being proportional to the squareroot of the initiator concentration. For example, if the refluxcondenser doubles the heat removal capacity of system, the catalystconcentration can then be increased fourfold.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view of the exterior ofapparatus adapted to the performance of the process of this invention.

FIG. 2 is a cross section of the apparatus shown in FIG. 1.

The apparatus comprises a shell-and-tube reflux condenser mounted on aconventional vinyl chloride polymerization reactor 2. The reactor isusually constructed with a steel outer wall and a glass inner lining andhas a jacket surrounding the reactor for cooling or heating purposes.The reactor may vary in capacity from about 2,000-l0,000 gallons. Thecooling surface provided by the cooling jacket for a reactor of 5,000gallons constitutes about 350 sq. feet of surface. As will be evidentthe heat transfer from the outside surface of the reactor to the insidesurface of the glass lined reactor is not good, and the heatconductivity ranges generally when clean and during polymerization fromabout 30-50 B.t.u. per hour per sq. foot per degree Fahrenheit. Thecondenser is of the tube type and can advantageously be constructed tohave a cooling surface area of 1 to 2 times the cooling surface area ofthe reactor.

The charge to reaction vessel 2. is introduced through an opening at thetop not shown in the drawing and comprises the major components, vinylchloride monomer and water. The monomer may be entirely vinyl chlorideor vinyl chloride monomer together with a lesser amount of compoundscontaining a vinyl group such as, for example vinyl acetate, acrylicacid or the esters of acrylic acid or vinylidene chloride or vinylbromide, etc. Demineralized water, the other major component of thecharging material is added in an amount approximately equal to theamount of vinyl chloride monomer, but this amount may vary from about 30percent to about 70 percent by weight water based on total charge,dependent on reactants, operating conditions, product, etc.

Normal practice is to leave a vapor space above the liquid level of thecontents in the reactor. In accordance with this invention the vaporspace need not be increased to minimize fouling caused by carryover ofpolymer into the reflux condenser. Merely as illustrative, a reactorhaving a volume of 5800 gallons would be filled with approximately 5,000gallons liquid leaving a vapor space with a volume of about 800 galloncapacity above the liquid. In the preferred method of operation, some-40 percent of the water may be withheld and later introduced throughthe reflux condenser, preferably as a generally continuous spray, forthe purpose of forming a barrier and preventing polymer formation on theinner surface of the tubes of the reflux condenser. This has anadditional advantage in that a lower level is used at the initiation ofthe operation which minimizes the chances of carryover of entrainedpolymer particles into the reflux condenser.

The polymerization catalysts and additives are well known in the art andwill vary dependent upon the type of process employed and productdesired. One commonly used process is the emulsion or dispersionpolymerization which employs a redox catalyst system such as hydrogenperoxide with sodium formaldehyde sulfoxylate or potassium persulfatewith sodium bisulfite and dispersing agents such as the alkyl sulfatesand sulfonates. In the other commonly used system known in the art assuspension polymerization the catalyst may be isopropylperoxydicarbonateor caprylyl peroxide. Suspending agents such as polyvinyl alcohol orgelatin are usually employed. Suspension polymerization presentsdifficulties with respect to the use of a reflux condenser in that thelittle particles of solid material may be entrained and carried over andthe deposition of these solid particles on the walls of the refluxcondenser results in rapid clogging up of the condenser tubes. In theemulsion type of operation, entrained latex can also have a strongtendency to gum up the tubes of the reflux condenser. In situpolymerization of vinyl chloride monomer can also occur on the condensertubes in both types of polymerization.

Cooling water for contact with tubes 6 in condenser 1 may be introducedthrough pipe 12 entering the top of condenser 1 and terminating innozzle 13. Other suitable means such as introduction of water at aplurality of spaced points above the tubes may also be employed, butspraying is the preferred method of operation. The demineralized watersprayed from nozzle 13 flows down along the interior surfaces of thetube 6. Ordinarily 1-7 gallons water per minute, depending on size ofcondenser, is adequate to furnish all the water required for maintaininga coating on the interior surfaces and preventing polymer depositionalthough more may be used if desired. The effect of this small amount ofwater is indeed surprising when one considers the relatively largeamount of condensate, about 10-100 gallons per minute depending oncondenser size and polymerization rate, flowing from the condenser.After passing down through the tubes, the demineralized water falls fromthe tubes with the vinyl chlorides condensate into reactor 2.

The rate of flow of both the demineralized water and the liquidcontaining the defoamer is preferably at least as great as the rate ofremoval of normal volume shrinkage of the reaction mixture in thereactor. The volume of the reaction mixture normally shrinks during thecourse of the polymerization because of the difference in densitiesbetween the monomer and the polymer. The rate of flow can beadvantageously greater than the rate of batch shrinkage and is limitedonly by the volume of the reactor above the liquid level of the reactionmixture.

In certain instances it is advantageous to contact the vinyl chloridevapor as it is withdrawn from the reactor particularly when effectingemulsion polymerization, with a dilute liquid spray defoamer. The liquidcontaining the defoamer is preferably water, but can be any inertdiluent. The defoamer can be any conventional defoamer which does notinterfere with the polymerization reaction. Since defoamers are normallyemployed in connection with vinyl chloride polymerization, suchdefoamers are well known in the art. The amount of defoamer employed inaccordance with this invention can vary over a wide range, depending onthe efficiency of the particular defoamer employed and the particularpolymerization formulation used. For example, polymerization systemscontaining high levels of emulsifier with consequent particular tendencyto form require a higher level of defoamer addition. The defoamer isadded to the reaction mixture at a rate such that the total amount ofdefoamer added is between 0.001 and 0.2 percent, preferably 0.005 and0.05 percent, by weight based on the weight of the reaction mixture. Theamount of liquid sprayed with the defoamer may be any amount whichconveniently provides a spray extending over the cross sectional area ofthe conduit through which the vinyl chloride vapor flows from thereactor to the condenser.

For convenience, the cooling water flowing through the shell portion ofthe condenser 1 can be the same as the water leaving the heat-exchangejacket (not shown) of the reactor 2. This water is normally at atemperature of about 25 to 40 C. dependent upon heat removalrequirements. Naturally as the water temperature is reduced the heatremoval capacity of the system is increased.

The temperature of the sprays is not critical and it is most convenientto use water at ambient temperature. However, using water colder thanambient temperature results in greater heat removal capacity and atendency for less polymer deposit.

If the vinyl chloride monomer contains noncondensable inert gases, suchas nitrogen, dissolved therein, it may be necessary to open valve 8intermittently during the first hour of polymerization in order topermit the gases to escape.

The temperature and pressure employed are those commonly used in thepolymerization of vinyl chloride and will vary somewhat depending uponthe product desired. Although polymerization may be carried out attemperatures ranging from about 0 C. to C. the commonly used temperaturerange varies from about 40 to about 75 C. The pressure may vary widelybut generally will be from about '1 00 to 250 lbs. per sq. inch andpreferably ranges from about 100 to lbs. per sq. inch. Usually thepressure unless otherwise regulated will be approximately theautogeneous pressure resulting from vaporization of the monomers in thereaction mixture.

Initially to raise temperature of reaction ingredients to the desiredtemperature for polymerization a heating medium i.e., hot water, ispassed through the jacket surrounding reactor 2 raising the temperatureof the reactants in the reactor. Once the reaction is initiated itbecomes highly exothermic and generates a great deal of heat.Consequently, a cooling medium i.e., cold Water is passed through thejacket and condenser to regulate and maintain the reactants in reactor 2at a substantially constant polymerization temperature.

As the reaction proceeds, vinyl chloride vapors are generated and riseupwardly from the top of reactor 2 through conduit 3 into condenser 1.Conduit 3 is provided for conveying vinyl chloride vapor from thereactor 2 to the condenser 1 and situated in conduit 3 is pipe 4terminating in nozzle 5. Liquid containing a defoamer may be sprayedfrom the nozzle 5 countercurrent to the flow of vinyl chloride vapor.The spray from nozzle 5 prevents entrained foam from rising intocondenser 1.

After passing through conduit 3, the vinyl chloride vapor enters thetubes 6 of the reflux condenser 1 wherein it is cooled and condensed.The liquid condensate falls from the tubes to the bottom of thecondenser 1, from which it flows into the reactor 2.

A conduit 7, through which gases may be vented, extends from the top ofthe condenser. A valve 8 controls the flow of gases through the conduit7.

Cooling water flows through the shell portion of the condenser 1,entering at inlet 9 and exiting at outlet 10. Baffles 11 direct the flowof cooling water through the condenser.

In an alternative method, the conduit 3 need not be employed andinstead, a single conduit may be used for both the upward passage ofvinyl chloride vapor and the downward return of the vinyl chlorideliquid condensate. The conduit connecting the top of the reactor 2 withthe condenser 1, should, of course, be sufficiently large to permitready flow of the vapors of vinyl chloride upwardly countercurrent tothe downward flow of the liquid condensate into the body of liquid inreactor 2.

A pipe 12 enters the top of the condenser 1 and terminates in a nozzle13 above the tube 6. Demineralized water is sprayed from the nozzle 13and flows down along the interior surfaces of the tubes 6. Theintroduction of water in contact with the inner surface of the tubes hasthe effect of forming a barrier or shield which prevents formation andaccumulation of polymer on the surface of the condenser tubes. Thereason for this inhibiting action is not known particularly when viewedin the light of the small amount of water relative to the quantity ofreflux condensate which comes from the condenser tubes. It is believedthat preferential wetting of the tubes with water occurs to form a skinwhich shields the surface of the tube from polymer formation. Theinternal condenser surface was maintained free of polymer for longperiods of time by the use of introduction of water in commercial sizeequipment.

The following examples illustrate the present invention with Example 1demonstrating the operation in an emulsion polymerization and Example 2demonstrating the operation in a suspension polymerization.

EXAMPLE 1 A shell-and-tube reflux condenser was mounted on aconventional 5,000 gallon vinyl chloride polymerization reactor. Thecondenser contained 235 one-inch tubes having a heat-transfer surface ofabout 500 sq. feet. The reactor was employed to produce numeroussuccessive batches of vinyl chloride polymer by conventional emulsionpolymerization. A typical reaction mixture consisted initlallyDemineralized water gallons 2,400 Vinyl chloride pounds 18,500 Seedlatex solids) do 33 Sodium formaldehyde sulfoxylate do 2.4

Polymerization was initiated at about 52 C. by continuously adding tothe reaction mixture an aqueous solution of 0.3 weight percent hydrogenperoxide and an aqueous solution of 1.5 weight percent of sodiumformaldehyde sulfoxylate. The solutions were added at an hourly rate of5 gallons and 3.7 gallons, respectively, for the first two hours.Thereafter, the rate of addition of each solution was reduced to about2.5 gallons per hours. An aqueous solution of 10.5 weight percent sodiumlauryl sulfate was also continuously added at a rate of from 8 to 11gallons per hour until a total of about 70 gallons had been added.

After polymeriztaion had been initiated, the temperature of the coolingwater in the heat-exchange jacket surrounding the reactor was adjustedto between 35 and 40 C. In order to maintain a polymerizationtemperature of about 52 C. The cooling water leaving the jacket was fedto the shell portion of the reflux condenser. The condenser was ventedabout three times during the first hour of polymerization to permitinert gases to escape. Each venting was carried out for about 5 minutesand the interval between each venting was about 15 minutes. Condensationof vinyl chloride in the reflux condenser began about an hour afterpolymerization had been initiated.

About three hours after polymerization had been initiated, and whilecondensation is occurring demineralized water was sprayed over thecondenser tubes and water containing a defoamer dissolved therein wassprayed counter-current to the flow of vinyl chloride vapor entering thecondenser. The demineralized water was sprayed at a rate of about 3gallons per minute; and the defoamer solution was sprayed at a rate ofabout 4 gallons per hour. The defoamer solution was prepared bydissolving one gallon of a commercially available active silicaderivative defoamer into 25 gallons of demineralized water.

Between 6 and 7 hours after polymerization had been initiated, thepressure in the reaction vessel decreased by 5 to 10 p.s.i., indicatingthat the polymerization was approaching the desired degree ofconversion. At this point addition of the hydrogen peroxide and sodiumformaldehyde sulfoxylate solutions was terminated. About 30 minuteslater, the system was vented and all spraying was stopped. The degree ofconversion for each batch was about to percent.

The average polymerization time when the reflux condenser was used was6.6 hours. In comparison, the average polymerization time for the samedegree of conversion when the reflux condenser was not used was 16.6hours. This substantial reduction in polymerization time is attributableto the increased heat-removal capacity provided by the reflux condenser.The quality of the product obtained using the reflux condenser was atleast as good as the product obtained without using the refluxcondenser.

Due to the spraying of the tubes and the entering vinyl chloride stream,about 40 batches of vinyl chloride polymer could be produced beforecleaning of the tubes in the condenser was required. In comparison, whenone of the spraying steps is omitted, the tubes must be cleaned after 10or fewer batches.

This example illustrates the benefits of carrying out the spraying stepsof this invention during at least a major portion of the polymerization.This portion is preferably the latter portion of the polymerization, butgreater reduction of polymer deposit is achieved when the spraying stepsare carried out continuously during the entire polymerization,especially when an emulsifier is added to the initial polymerizationmixture.

EXAMPLE 2 The same apparatus as used in Example 1 comprising ashell-and-tube reflux condenser mounted on a 5,000 gallon polymerizationreactor was employed for carrying out several conventional suspensionpolymerization operations.

A typical reaction mixture consisted initially of Demineralized watergallons 2,050 Vinyl chloride pounds 16,000 Suspending agent Polyvinylalcohol do.. 10 Additive buffer-Disodium phosphate do 4 Defoamer colloid581-B Active silica derivative defoamer gallon 1 Catalystisopropylperoxydicarbonate pounds 6.25

The demineralized water is first introduced into the reactor at atemperature of about 75-80 C. along with the suspending agent, bufferand defoamer. The reactor is then closed and a vacuum is drawn to removeresidual oxygen and noncondensables. This is followed by theintroduction of cold vinyl chloride monomer. At this point thetemperature of the reaction mixture is near the desired polymerizationtemperature of 59 C. and the catalyst is injected as a solution insolvent.

Immediately after the catalyst charge, the spraying of water onto thecondenser tubes is initiated at a rate of about 3 g.p.m. and ismaintained during the condensation period, preferably untilpolymerization end. Concurrent with the start of water spray the jacktwater temperature is automatically reduced as required to remove theexothermic heat of polymerization and to maintain the reaction mixtureat a temperature of 59 C.

By -30 minutes after polymerization had been initiated, the rate ofexotherm from the reactor and condenser exceeded that which couldordinarily have been removed by the reactor jacket alone. Yet at thispoint only a 35-40 C. jacket water temperature was required to maintaina constant polymerization temperature.

About 3 to 3 and /2 hours after polymerization was initiated, a sharpreduction in reactor pressure occurred indicating the nearly completeconversion of the monomer to polyvinyl chloride. The polymerization wasthen terminated by the venting of unreacted monomer from the reactor.

By the end of polymerization about 900 gallons of demineralized waterspray had been introduced through the condenser. Most of this waterwould ordinarily have been charged initially to the reactor. Nocontinuous spray of defoamer was required for the suspensionpolymerization.

Eleven batches were produced in the above manner. Polymerization timeaveraged about 3.5 hours whereas polymerization time when the refluxcondenser was not used could not be reduced below eight hours.

The condenser tubes were inspected after each batch and after the totaleleven batches were produced there existed no significant deposition onthe condenser tubes. In comparison a similar operation was conductedwithout the spraying of water onto the condenser tubes and the overallheat transfer coefiicient of the condenser reduced steadily throughout asingle polymerization from 100 to 10 B.t.u. per hour per sq. foot perdegree Fahrenheit due to deposition of polymer particles on the tubesurfaces.

We claim:

1. In a process for polymerizing vinyl chloride in an aqueous mediumwithin a reactor wherein vinyl chloride vapor is withdrawn from thereactor, condensed in a condenser, and returned as a liquid to thereactor, the improvement which comprises spraying a coating of water onthe interior surfaces of the condenser during the condensation period,whereby deposit of vinyl chloride polymer on such interior surfaces issubstantially reduced.

2. A process as claimed in claim 1 wherein the Water is introduced intothe condenser in the form of a spray.

3. A process as claimed in claim 1 wherein the water is introduced intothe condenser at the rate of about 1 to 7 gallons water per minute.

4. A process as claimed in Claim 1 wherein a portion of the watercomponent of the aqueous medium, rather than being introduced into thereator before said process is initiated, is introduced into thecondenser as a generally continuous stream during vaporization andcondensation of vinyl chloride vapor.

5. A process as claimed in Claim 4 wherein 20-45% of the total aqueousmedium is introduced into the condenser as a water spray during thepolymerization process.

6. A process as claimed in Claim 1 wherein water is sprayed in contactwith and counter-current to the flow of the vinyl chloride vapor as saidvapor flows from said reactor into said condenser.

7. A process as claimed in claim 6 wherein the water introduced incontact with the vinyl chloride vapor contains a defoamer.

8. In a process for polymerizing vinyl chloride in an aqueous mediumcontaining a suspension of solid vinyl chloride polymer within areactor, and wherein vinyl chloride vapor is withdrawn from the reactor,condensed in a condenser, and returned as liquid to the reactor, theimprovement which comprises generally continuously coating the interiorsurface of the condenser with water during a major portion of thepolymerization period, whereby deposit of vinyl chloride polymer on suchinterior surface is substantially reduced.

9. In a process for polymerizing vinyl chloride in an aqueous mediumcontaining a dispersing agent to produce a latex or dispersion of veryfine solid polymer particles within a reactor, wherein vinyl chloridevapor is withdrawn from the reactor condensed in a condenser by exposureto the interior walls thereof, and returned as a liquid to the reactor,the improvement which comprises continuously directing a coating ofwater on such interior walls of the condenser, whereby deposit of vinylchloride polymer on such walls is substantially reduced.

10. A process as claimed in claim 1 wherein the vinyl chloride vapor isreleased through an opening in the top of the reactor and the liquidcondensate from the condenser returns to the reactor through the sameopening.

11. A process as claimed in claim 1 wherein the cooling surface area ofthe condenser is between 1 to 2 times the area of the cooling surface ofthe reactor.

References Cited UNITED STATES PATENTS 3,515,709 6/1970 Nelson et al.26092.8

3,669,946 6/1972 Koyanagi et al. 26087.5 R

3,578,649 5/1971 Badquerhanian et al 260-928 FOREIGN PATENTS 4,839,9296/1952 Canada.

JOSEPH L. SCHOFER, Primary Examiner J. KIGHT, Assistant Examiner US. Cl.X.R. 260-863, 87.1, 87.5

